Various types of visual displays are used in connection with electronic devices. A television, for example, uses a cathode-ray tube (CRT), where a directed beam of electrons selectively excites phosphors in the screen, producing a multitude of variously-colored picture elements (pixels), that collectively form an image. Light-emitting diodes (LEDs) are also common, though far more limited in their ability to display complex images. Although somewhat more difficult to manufacture, liquid-crystal displays (LCDs) are gaining popularity because of their image-producing versatility and low-power consumption.
In general, LCDs are composed of a liquid-crystal layer sandwiched between transparent light-polarizing materials, along with electrical conductors and electrodes that enable a bias voltage to be applied across a specific small area (that is, a pixel) of the liquid-crystal layer. Applying the voltage difference to the pixel electrode alters the light-polarizing characteristics of the liquid crystal material proximate to the electrode. Light waves that are polarized when passing through one polarizing layer will typically not pass through the other, cross-polarized layer, unless the phase angle of the polarized light is changed as it passes through the liquid-crystal layer between them. Liquid crystals are substances that flow like liquids, but whose molecules nevertheless maintain a definite orientation with respect to each other. This orientation may be changed from one that causes the needed phase-angle change to one that does not, through the application of an electrical charge, as described above. The liquid-crystal orientation, therefore, determines whether the pixel will appear light or dark.
In an LCD, the light that produces the image itself is not created by the liquid crystals, but is supplied by separate light-sources such as LEDs or reflected ambient light. LCDs create images by determining where light will be allowed to pass through the LCD assembly and where it will be absorbed. In this sense, it is more appropriate to say that a portion of the liquid-crystal material is “activated” by the applied voltage, rather than illuminated. The amount of this light that is allowed through can be controlled very specifically by adjusting the level of the applied voltage. This means that the pixel can be adjusted to one of many finely varying levels of brightness. Color LCDs operate by employing three independently-controllable sub-pixels for each display pixel. Depending on the individually applied voltage, the sub-pixels filter out varying amounts of red, green, and blue light, respectively, to produce the different-colored portions of a displayed image. The color of each image pixel is determined by the intensity of light permitted to pass through its colored sub-pixels.
The liquid-crystal activating voltage potential can be supplied to the pixel in different ways. The simplest uses a transparent, conductive backplate (or plane). Smaller appropriately-shaped electrodes on the transparent front plate form the opposite charge plates that can be selectively turned on and off. This arrangement is satisfactory for calculator displays and the like where only a limited number of shapes and letters such as numerals or letters will need to be formed by combining the individual elements, such as numerals or letters.
More advanced LCDs use a grid of conductor rows and columns to activate selected pixels or sub-pixels. More than one pixel may be simultaneously activated by this row and column matrix, although a complete image cannot usually be created in this way. Multiplexing may be used, however, to create the proper combination of light and dark pixels. In this case, a pre-determined number of pixels are activated in each of a number of sequential steps. The speed at which alternating pixel groups are activated should be sufficient to produce an image detectable by the human eye. In addition, a capacitor may be associated with each pixel, allowing it to retain some charge even when it is not actively connected to the voltage source.
LCDs can now be found on many electronic devices. For example, modern video camera-recorders (camcorders) often include an integrated LCD video display. Many camcorders include an optical or electronic viewfinder as well, although many users prefer to watch the LCD while they are recording because it provides the most representative image of what is being captured.
Camcorders, being portable, are usually battery-powered (although they may be able to use other power sources when available). The batteries are of a type, for example nickel-cadmium, that can be repeatedly recharged. An actual “change” of batteries is, therefore, not regularly required under normal operating conditions. The amount of time that a user can operate the camcorder between battery recharges is of some importance, however. Since the camcorder is portable it is often carried to locations remote from alternate power sources. Once the batteries are discharged, the camera is inoperable until they can be recharged. One or more extra charged batteries can be carried, of course, but doing so imposes somewhat of an inconvenience. And, of course, any extra batteries will eventually discharge below operating power levels as well.
In the future, (and even to some extent in the present) complex LCDs will also be found on mobile telephones and personal digital assistants (PDAs). Such devices are and will continue to be used to provide wireless access to public and private communications networks such as the Internet. Through one of these devices, a user can connect to the network and download various text and graphic files from, for example, Web servers. The files' content can then be viewed on the LCD. These portable wireless devices create even more severe power-consumption restrictions because of their small size. No mobile phone the size of a camcorder would today be commercially accepted, and so ever-smaller batteries are being required to function for an ever-increasing time between charges.
It is, therefore, advantageous to design as many power-conservation features as possible into battery-powered devices, such as mobile phones, PDAs, and camcorders. Several such features already exist. Perhaps the most simple is an on/off switch, which allows the user to select a mode that consumes no power (or almost none). The device also may automatically shut itself off, or, alternately, turn off only selected power-consuming operations, after a certain pre-determined period of non-use.
The LCD display, in spite of its power-consumption advantage, still consumes a significant amount of power. One way to conserve display power, of course, is by shutting down the display itself when it is not in use—even if other (non-display) operations are continuing. This may even be done automatically, for example by turning on the display only when a motion detector detects the user's presence, or turning it off when a low-power state is detected. Other power-saving approaches make use of ambient light when available to back light the LCD and produce a brighter image without consuming extra battery power.
What is needed, however, is a power-conservation feature that can be selectively used to reduce LCD power consumption in battery-powered devices such as mobile phones and camcorders, regardless of available ambient light, and yet allow the user to continue utilizing the display for its intended function. The present invention provides just such a solution.